The present invention relates to a process for producing plate-like barium ferrite particles by substituting a part of Fe(III) of the plate-like barium ferrite particles with Ti(IV) and a divalent metal selected from the group consisting of Co, Ni, Zn and Mn. More in detail, the present invention relates to a process for producing plate-like barium ferrite particles represented by the formula(I): EQU BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 (I)
wherein x and y are respectively 0.3 to 1.2 and M is a divalent metal selected from the group consisting of Co, Ni, Zn and Mn for magnetic recording, which comprises autoclaving an aqueous highly alkaline suspension containing particles represented by the formula: Fe.sub.12-x-y M.sub.x Ti.sub.y O.sub.16, wherein x, y and M are respectively the same as defined above, having a spinel crystalline structure and an average particle size of less than 0.1 .mu.m and Ba ions at a temperature in the range of from 250.degree. to 330.degree. C. so that the atomic ratio of Ba to total amount of the metals is in a range of from 1:8 to 1:10.
In recent years, accompanying with the propagation of VTR, audio component, word processer and computer, non-acicular ferromagnetic particles of a favorable dispersibility provided with a suitable coercive force(Hc) have been required as the magnetic material for recording, particularly as the magnetic material for the perpendicular magnetic recording.
In general, as the ferromagnetic non-acicular particles, barium ferrite particles have been well known. However, the coercive force of barium ferrite particles obtained by the dry method is ordinarily larger than 3,000 Oe, and because of such a high coercive force, barium ferrite particles are not favorable as a magnetic material for magnetic recording.
Accordingly, in order to reduce the coercive force of the barium ferrite particles, various methods are proposed. For instance, it has been known for a long time that a part of Fe(III) in the barium ferrite particles is substituted by a divalent metal such as Co(II) and Ti(IV).
Further, it is necessary that the value of magnetization of the magnetic particles is as large as possible, and its necessity is clearly seen in the description in Japanese Patent application Laying Open No. 56-149328 (1981) that it is required that the saturation magnetization of magnetoplumbiteferrite used as a material for the medium of magnetic recording is as large as possible.
On the other hand, as a method for producing barium ferrite particles, a method of treating an aqueous alkaline suspension containing Ba ions and Fe(III) in an autoclave as a reactor (the method is hereinafter referred to as "the autoclaving method") has been hitherto known, and by selecting the reaction conditions in the autoclaving method, barium ferrite particles precipitate. The thus precipitated particles are hexagonal plate-like particles, and the distribution of the particle size and the average size of the particles differ according to the reaction conditions resulting in the difference of the magnetic properties of the particles. In the technical field of producing plate-like barium ferrite particles for use in magnetic recording by means of the autoclaving method, not only the method of formation of the product from an aqueous solution but also the method of formation of the product from a fluid has been hitherto tried.
However, in the case of the method of formation of the product from a fluid, it is necessary to carry out the autoclaving treatment at a temperature of higher than 400.degree. C.
On the other hand, a technique by which non-ferromagnetic barium ferrite particles are formed from an aqueous solution according to the autoclaving method and the thus obtained non-ferromagnetic barium ferrite particles are sintered at a high temperature to obtain ferromagnetic barium ferrite particles has been tried.
In consideration of the above-mentioned process, in order to effectively reduce the coercive force without remarkably reducing of the value of magnetization of the magnetic particles by substituting a part of Fe(III) in barium ferrite particles with a divalent metal, M(II) and Ti(IV), the present inventors have repeated the systematic investigation on the relationship between conditions in the formation of various plate-like barium ferrite particles from an aqueous suspension by autoclaving the aqueous suspension containing both the particles represented by the formula, Fe.sub.12-x-y M.sub.x Ti.sub.y O.sub.16, having a spinel crystalline structure and barium ions while variously changing the condition in formation thereof such as the atomic ratio of M(II) and Ti(IV) to Fe(III), the atomic ratio of Ba to Fe(III), M(II) and Ti(IV), the particle size of the ferrite particles, the concentration of alkali in the aqueous alkaline suspension, the reaction temperature of autoclaving method, the reaction time and the stirring condition of the aqueous suspension, and have studied the process for directly producing the plate-like barium ferrite particles represented by the formula (I) from the aqueous suspension by autoclaving the aqueous highly alkaline suspension, and as a result, the present inventors have found that the magnetic properties of the thus obtained plate-like barium ferrite particles were in close relationship to the composition of the particle, the particle shape, the average particle size and the particle size distribution.
Namely, it has been found by the present inventors' study that in the case of autoclaving an aqueous highly alkaline suspension containing both the particles represented by the formula: EQU Fe.sub.12-x-y M.sub.x Ti.sub.y O.sub.16
wherein x and y are respectively 0.3 to 1.2 and M is a divalent metal selected from the group consisting of Co, Ni, Zn and Mn, having a spinel crystalline structure and an average particle size of less than 0.1 .mu.m, and Ba ions at a temperature of 250.degree. to 330.degree. C. so that the atomic ratio of Ba to total amount of metals is in a range of from 1:8 to 1:10, plate-like barium ferrite particles represented by the formula (I): EQU BaFe.sub.12-x-y M.sub.x Ti.sub.y O.sub.19 (I)
wherein x and y are respectively 0.3 to 1.2 and M is divalent metal selected from the group consisting of Co, Ni, Zn and Mn, showing a magnetization of larger than 35 emu/g in a magnetic field of 10 KOe, a coercive force of 500 to 1000 Oe, for magnetic recording are directly formed, and based on the finding, the present invention has been attained.